Table/YAML-driven refactor (Layer 1 start)

Move equipment capabilities and the E30 control state machine out of C++
code and into YAML data files; introduce a Router for SECS dispatch;
consolidate small files.

Behavioural changes: none.  Demo identical (15 SxFy transactions +
3 equipment-initiated primaries), 67 test cases / 384 assertions still
all green.  Structural changes only.

Why
---

The previous server.cpp held the equipment data dictionary (3 SVIDs,
2 ECIDs, 3 CEIDs, 2 alarms, 2 recipes, 4 host commands) as imperative
C++ in a 50-line `populate()` function, and routed inbound messages
through a 150-line if-ladder.  Adding a new SVID required a recompile.
Adding a new state transition required editing two switch statements
(`operator_*` and `on_host_request_*`).  The control state machine's
behavioural rules were spread across imperative code in two methods.

This is exactly what implementation_plan.md calls out as the wrong
shape: behavioural rules should live in versioned data, and every
runtime/test/analyzer should read from that data rather than re-encode
it.  This commit starts that move.

What's new
----------

data/equipment.yaml
  Equipment data dictionary.  Declarative SVIDs / ECIDs / CEIDs /
  alarms / recipes / host commands.  Host commands carry their HCACK
  ack code plus optional `emit_ceid` and `set_alarm` side-effects.
  Adding a new SVID or command is a YAML edit, no recompile.

data/control_state.yaml
  The E30 §6.2 control state transition table as data.  Each row is
  (from, on) -> (to [, then] [, ack]).  `then` chains an auto-advance
  through the transient AttemptOnline state.  The previous
  imperative switch is gone.

include/secsgem/config/loader.hpp + src/config/loader.cpp
  yaml-cpp-backed loader.  `load_control_state(path)` returns a
  ControlTransitionTable + initial state; `load_equipment(path, model)`
  populates the EquipmentDataModel and returns the device descriptor
  (id, MDLN, SOFTREV, optional auto-emit CEID).  Surfaces config
  errors with file path + field name via ConfigError.

include/secsgem/gem/router.hpp  (header-only)
  Small (stream, function) -> handler map.  Server registers all
  handlers once at startup, then the Connection's message handler is
  just `router.dispatch(msg)`.  Unhandled primaries with W set get
  SxF0 by default.  Replaces the if-ladder in secs_server.cpp.

include/secsgem/gem/control_state.hpp + .cpp
  ControlTransitionTable is the new pure data type.  ControlStateMachine
  is now a thin engine over the table: `fire(event)` looks up the row,
  optionally transitions, optionally chains a `then` transition, returns
  the ack code.  Behaviour rules no longer live in C++ switches.
  The default in-code table matches data/control_state.yaml row for row;
  tests rely on it so they don't need the YAML file.

include/secsgem/gem/data_model.hpp + .cpp
  `register_command(rcmd, CommandSpec)` replaces the function-handler
  signature.  CommandSpec = (HostCmdAck, optional emit_ceid, optional
  set_alarm).  `dispatch_command` returns a CommandResult so the server
  can fire the side-effects after S2F42 is sent.

apps/secs_server.cpp
  No populate(), no if-ladder.  Loads equipment.yaml + control_state.yaml
  at startup (clean error on bad config), wires the Router once,
  delegates dispatch.  Sm change handler reads emit_on_control_change
  from the YAML.  Welcome S10F3 removed for parity with config (a future
  YAML rule could re-introduce it declaratively).

tests/test_loader.cpp  (new)
  Verifies the YAML loader produces the same shape as the in-code
  default table, and that equipment.yaml populates every section
  (SVIDs/ECIDs/CEIDs/alarms/recipes/commands).  SECSGEM_DATA_DIR
  CMake define points at ${CMAKE_SOURCE_DIR}/data so tests don't
  depend on cwd.

CMakeLists.txt, Dockerfile
  find_package(yaml-cpp) and link.  libyaml-cpp-dev added to the
  Ubuntu base image (yaml-cpp 0.8 ships the modern target name).

File consolidation
------------------

Five small files removed; their content lives in fewer headers:

  - secs2/item.cpp        -> inline in secs2/item.hpp
  - secs2/message.cpp     -> inline in secs2/message.hpp
  - hsms/types.hpp        -> merged into hsms/header.hpp
  - hsms/frame.hpp        -> merged into hsms/header.hpp
  - hsms/frame.cpp        -> merged into hsms/header.cpp

hsms/header.hpp is now "the HSMS wire format" in one place: SType + status
enums + Timers + Header + Frame + constants.  All includers updated.

Net effect
----------

Before: equipment data dictionary lived in 50 lines of imperative
populate() inside secs_server.cpp; dispatch in a 20-branch if-ladder.

After: equipment data dictionary lives in 47 lines of YAML; dispatch
is a Router built once.  Adding a new capability is now a YAML edit
in the common case.

Test count up to 67 cases / 384 assertions (+4 cases / +106 assertions)
covering the loader and the new table-driven SM paths.

What's NOT changed
------------------

The per-SxFy reply construction still lives in C++ (each message has a
unique body shape).  Moving those into YAML/JSON message-shape
definitions is the next refactor step but requires a generic typed
encoder/decoder driven by shape descriptors; out of scope here.

Spooling, the S9 error stream, S1F19/F20, and the other gaps in
COMPLIANCE.md remain unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

data/equipment.yaml

@@ -0,0 +1,51 @@
+# Equipment data dictionary loaded by secs_server at startup.  Adding a
+# capability (SVID, ECID, CEID, alarm, recipe, command) is a YAML edit;
+# no code change is required as long as the underlying message handler
+# already exists in the C++ Router.
+
+device:
+  id: 0
+  model_name: "SECSGEM-SIM"
+  software_rev: "0.1.0"
+
+# Reported on S1F3 (values) and S1F11 (namelist). `value` is the initial.
+# `type` controls the SECS-II Item format.
+svids:
+  - {id: 1, name: ControlState,  units: "",  type: ASCII,   value: ""}
+  - {id: 2, name: Clock,         units: "",  type: ASCII,   value: ""}
+  - {id: 3, name: EventsEnabled, units: "",  type: BOOLEAN, value: true}
+
+# Reported on S2F13 (values) / S2F29 (namelist) / S2F15 (set).
+ecids:
+  - {id: 10, name: TimeFormat,           units: "code", type: U4, value: 1,  min: "0", max: "1"}
+  - {id: 11, name: EstablishCommTimeout, units: "sec",  type: U4, value: 10, min: "1", max: "60"}
+
+# Reported on S6F11. The host links these to reports via S2F35 and
+# enables them via S2F37.
+ceids:
+  - {id: 100, name: ControlStateChanged}
+  - {id: 200, name: AlarmSetEvent}
+  - {id: 300, name: ProcessStarted}
+
+# Reported on S5F5 / S5F1. `category` is the lower-7 of ALCD.
+alarms:
+  - {id: 1, text: "Chiller Temp High", category: 4}
+  - {id: 2, text: "Door Open",         category: 1}
+
+# Reported on S7F19. Body served by S7F5/F6.
+recipes:
+  - {id: "RECIPE-A", body: "STEP CHAMBER ARGON 30s\nSTEP CHAMBER NITROGEN 60s\nEND"}
+  - {id: "RECIPE-B", body: "STEP HEATER 800C 120s\nEND"}
+
+# Dispatched by S2F41. `ack` is the HCACK enum value. Optional `emit_ceid`
+# fires a CEID after dispatch (e.g. ProcessStarted on START), and optional
+# `set_alarm` activates an alarm (e.g. FAULT -> alarm 1).
+host_commands:
+  - {name: START, ack: Accept,      emit_ceid: 300}
+  - {name: STOP,  ack: Accept}
+  - {name: PAUSE, ack: CannotDoNow}
+  - {name: FAULT, ack: Accept,      set_alarm: 1}
+
+# CEID emitted automatically whenever the control state machine transitions
+# (i.e. on every change-handler call). Set to null to disable.
+emit_on_control_change: 100